Control systems for furnaces are known. The heating capacity of these systems depends on the quantity of combustible fluid fed to the burner and on the ratio between this quantity and the combustion air fed to the burner. To obtain an optimal heating effect, an adjustment of the ratio between the fluid and the air is recommended. In known control systems, the air is conveyed to the burner through a connecting line or pipe by a blower having a constant rotational speed. A butterfly valve controlled by the regulator is used in the connecting line to control the air pressure. Control of the pressure adjuster for the combustible fluids fed to the burner is effected as a function of the air pressure.